1. Field of the Invention
This invention relates generally to cells, such as electrochemical cells, in which lithium is the active species, and more particularly to electrochemical cells having a molten salt electrolyte.
2. Prior Art
There is a great deal of current interest in better methods for energy storage. This is especially important for applications such as electric vehicles and the large scale storage of electric energy to level the load of stationary power plants. It does not appear that major improvements in important parameters, such as the amount of energy stored per unit weight or volume, at reasonable costs, can be expected from systems that employ aqueous electrolytes.
One of the developments currently being pursued involves a lithium-based cell, in which the negative electrode is a lithium alloy (typically either lithium-aluminum or lithium-silicon), the positive electrode is an iron sulfide, and the electrolyte is a molten salt, such as the eutectic composition in the lithium chloride-potassium chloride system. Because of the high melting point of such salts, such cells must be operated in the temperature range 400-500 degrees centigrade.
This requirement to operate at such high temperatures has several important disadvantages. One of these is that various degradation processes, such as corrosion of the cell container, seals, and other components are accelerated by such high temperatures. Another is that a substantial amount of energy is lost through heat transfer to the surroundings. Another is that the voltage obtained from such cells is lower at elevated temperatures, due to the fundamental property of the negative temperature dependance of the free energy of the cell reaction. Furthermore, the higher the temperature of operation, the greater the potential problems related to damage to the cell during cooling to ambient temperature and reheating, whether deliberate or inadvertent. Differences in thermal expansion, as well as dimensional changes accompanying phase changes, such as the freezing of the molten salt, can cause severe mechanical distortions, and therefore damage to cell components.
Accordingly, it would be advantageous to have cells of this type that operated at lower temperatures, particularly where the temperature of operation is near the boiling point of water, e.g., 100-150 degrees centigrade, rather than ambient temperature, as it would be easier to maintain the desired temperature by either heating or cooling.
Cells involving a lower temperature molten salt electrolyte have been investigated where the molten salt is based upon a solution of aluminum chloride and an alkali metal chloride. However, such salts are not stable in the presence of the respective alkali metals. As a result, an auxiliary solid electrolyte must be used to separate the alkali metal and the salt. One example of such a cell involves a molten sodium negative electrode, a solid electrolyte of sodium beta alumina, a molten aluminum chloride-sodium chloride salt, and either antimony chloride or an oxychloride dissolved in the chloride salt as the positive electrode reactant.
Such a cell can operate in the temperature range 150-250 degrees centigrade. It has the disadvantage of having to employ the solid electrolyte, which increases the cell impedance, as well as adding to the cost and complexity.